Radio frequency communication systems typically include one or more base stations and one or more subscriber units. As is known, each such base station and subscriber unit contains a receiver unit. It is known to arrange such a receiver unit to determine a quality value for the received signal of its current channel, as well as a quality value for the received signal for each of one or more adjacent channels. It is then possible to decide which channel to use based on comparing the quality values for the corresponding received signals. If a channel offering increased received signal quality is found (the "preferred channel"), the subscriber unit then leaves its current channel and moves to the preferred channel, thereby achieving a better received signal. This process is called a "hand-over".
In today's radio frequency systems, there are several ways to determine a quality value for a received signal. For example, a first approach is to determine the bit error rate. A second technique is to determine the total power of the signal, including the signal plus noise and interference.
Still another way to determine a quality value for a received signal is described in the aforementioned Birchler patent. Briefly, Birchler determines a received signal quality value by measuring the ratio of the signal to the sum of noise and interference. This technique allows the receiver unit to distinguish between a good channel with high signal power versus a bad channel with high interference.
A limitation of Birchler, however, is the maximum ratio of signal to noise-plus-interference-power is achieved even when the actual received signal quality is much better. As a result, a receiver unit cannot distinguish between a non-preferred channel offering a mediocre received signal quality and a preferred channel offering a substantially increased received signal quality. Because the receiver unit cannot identify the preferred channel, the receiver unit cannot take advantage of the substantially increased received signal quality offered by the preferred channel.
Another limitation of Birchler is that the received signal quality determination is dependent on the particular data symbols in the received signal. Thus, when certain data symbols are received, the received signal quality determination is lower than when other data symbols are received. This causes a large variance in the determinations and allows the receiver unit to misinterpret received signal quality. As a result, it is possible the subscriber unit will handover to a channel offering a degraded, rather than an improved, received signal quality.
Accordingly, there is a need for an improved method for a receiver unit to determine a quality value for a received signal.